GST is known as a detoxicating enzyme which catalyzes the conjugation (reaction) of various substances, which enter into an organism, or analogous substances (hereinafter referred to as substrates) which are produced in vivo, with glutathione of reduced type. Many of the substrates react with the in vivo protein or nucleic acid to cause a change to a morbid state (for instance, carcinogenesis) but the reactivity of these substrates are neutralized by said conjugation to be transformed into more water-soluble products and metabolized in the liver, etc. and finally excreted out of the body.
GST is found existing in various kinds of living species inclusive of mammals and is especially contained in considerable quantities in the liver, spleen, kidney, lung, brain, skeletal muscles, placenta, and small intestine, and also found in the skin, red blood cell, and white blood corpuscule though very small in amount. GST generally comprises various types of enzymes and is not only specific to species but also specific to the internal organs and tissues.
In the case of a human being, basic GST and acid GST are known (see The Journal of Biological Chemistry, vol. 259, No. 20, pp. 12,444-12,448 and pp. 12,449-12,455 (1984)). Basic GST has the isoelectric point (pI) of 7-9 and consists of two subunits, each subunit having a molecular weight of about 23,000. Acid GST has the isoelectric point (pI) of 4-5 and consists of two subunits, each subunit having a molecular weight of about 22,000. Basic GST exists mainly in the liver of a healthy normal adult and also in other places such as kidney, testicle, small intestine, brain, and lung though less in amount. While acid GST scarcely exists in the liver of a healthy normal adult but found existing in the liver of a newborn baby, placenta, such proliferous cells as liver cancer cells, stomach cancer cells, etc., red blood cell, and white blood corpuscule. Since acid GST produced by the proliferous cells is extricated out of the site of its production into the blood, it has possibilities of being utilized as a tumor marker highly specific to such digestive system cancers as liver cancer, stomach cancer, colon cancer, esophagus cancer, and cancer of the pancreas, and such blood tumors as cancer of the blood and lymphocytoma.